Phys Prac

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Outline Thomson’s experiment to measure the charge/mass ratio of an electron

Thomson assumed that the cathode rays consisted of negatively charged particles. Aim: To determine charge-mass ratio of the cathode ray particles. Method: 1. Thomson applied both magnetic field (provided by a current carrying coil) and electric field (provided by parallel charged plates) such that the cathode rays could pass through undeflected. Mathematically, FE = FB Eq = qvB v = E/B (We can sub in E=V/d  v = V/dB) This allowed the velocity of the cathode ray particles to be determined, since E and B are known. – –

Thomson found that cathode rays moved much slower than light Proved beyond any doubt that cathode rays are not a form of EM radiation

2. Thomson removed the electric field and applied only the magnetic field on the cathode ray. The cathode ray deflected under the influence of the magnetic field. By careful observation and measurement, he could measure the radius of curvature of the cathode ray particles. a. The deflection from the spot which the cathode ray would strike was measured b. Using the geometry of the apparatus, r was calculated. The magnetic field exerts a centripetal force on the beam causing it to deflect. Because of this phenomenon we can treat the magnetic force as an equivalent to the ray’s centripetal force. Mathematically,

FB = Fc qvB = mv2/r q/m = v/rB (We can sub in v = V/dB)

Since v, B and r are known, (are measurable quantities) the q/m ratio can be determined. – – –

Thomson found that the q/m ratio were constant regardless of the cathode material used. The constant q/m ratio confirms the particle nature of cathode rays. The q/m ratio for cathode rays was 1800 times greater than that of a hydrogen ion. (q/m = 1.76 x 1011 C/kg)

This was interpreted as: – –

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Atoms were made of smaller elementary particles: being the components of the cathode rays ie electrons q/m ratio of electron = 1800 x q/m ratio of hydrogen ion ○ charge of an electron is much greater than the charge of a hydrogen ion ○ mass of the electron is much smaller than the hydrogen ion

Perform an investigation to demonstrate and identify properties of cathode rays using discharge tubes: containing a Maltese cross, containing electric plates, with a fluorescent display screen, containing a glass wheel, and analyse the information gathered to determine the sign of the charge on the cathode rays

Aim: To determine some of the properties of the rays which come from the cathode of a discharge tube. Hypothesis: The properties of a cathode ray indicate it is a stream of negatively charged particles Risk assessment: Equipment Induction

Risk High voltage

Precaution Avoid touching the induction coil when it is switched on

coil

electricity

Discharge tube

X Ray radiation

Glass ware

Switch off all power supplies while the experiment is being set up (turn power on afterwards) Minimise operating voltage to lessen the energy of the X-rays and make it significantly safer Keep a fair distance away from the discharge tube when it is switched on Switch on discharge tube for short periods of time to make an observation Handle with care

Method: 1. The power pack was connected to the induction coil and set to 6V DC using connecting wires in a series circuit. 2. The discharge tube containing the maltese cross was then connected to the induction coil. The cathode of the induction coil was connected to the anode of the maltese cross discharge tube while the anode of the induction coil was connected to the cathode of the discharge tube. 3. When the power supply was switched on to produce a continuous spark from the cathode to the anode of the induction coil, the cathode ray tube holding the maltese cross was observed at the end of the tube for the effect of the cathode ray fired at the Maltese cross. These observations were recorded. 4. The discharge tube containing the maltese cross was then replaced with another discharge tube and steps 1 to 3 were repeated.

These tubes are: Type of discharge tube CRT containing electric plates CRT containing a rotating wheel CRT containing fluorescent screen

What to observe The effect of an electric field on the cathode rays The effect that the cathode ray has on the wheel when the tube is horizontal The effect of placing a set of magnets around the cathode rays.

Results: CRO Maltese cross Fluoresce nt screen

Observations A shadow of the maltese cross appeared on the glass opposite the cathode. Cathode ray was deflected and observed to bend in its path

Supports Light behaviour  travels in a straight line Using right hand palm rule  cathode ray is negatively charged

Paddle Wheel Electric plates

The paddle wheel turned when it was struck by the cathode ray Cathode ray was deflected by the electric plates towards the positive plate  when voltage in electric plates were increased, deflection increased.

Cathode rays possess momentum and energy Cathode ray is a negatively charged particle and is influenced by an electric field

What are the properties of cathode rays which can be deduced from this experiment? – – – – – –

Cathode rays travelled in straight lines  produce a shadow from maltese cross Cathode rays are deflected by electric and magnetic fields Cathode rays contain momentum (i.e. are able to turn a small paddle wheel placed in its path) and possess energy Cathode rays are negatively charged Cathode rays are unable to penetrate thick metals (i.e. maltese cross) They are produced by the negative electrode, or cathode, in an evacuated tube, and travel towards the anode.

Can we conclusively say that cathode rays are electrons? Why or why not? – – –

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Cathode rays are observed to obtain mass (momentum when hitting paddle wheel) They are deflected by both electric and magnetic fields and by observing their deflection it is seen that they are negatively charged In the past cathode rays could not be deflected by electric fields due to the lack of technology available. This was because cathode ray tubes in the past were not completely vacuumed. Therefore the gas particles in the tube ionised to form gas ions which lined up on the electric plates inside the tube. This neutralised the electric plates. In this experiment, equipment is used with a low gas pressure  allowed for deflection to be observed

Term Accurac y

Definition For measurement:  depends on the equipment used For a results:

Reliabili ty

 the degree to which the result is to the accept value or standard value (found in textbooks) First hand data:  if the experiment was repeated to give the same results  Consistency in results Second hand sources:  the trustworthiness of the source

Validity

First hand information and data:  How fairly tested the hypothesis is  All variables are kept constant apart from those being investigated

Strategies – Categorising information to integrate more detail into the material – Keep variables constant (other than dependant/independent variables) – Eliminate systematic errors by careful planning – Use more sensitive equipment

For experiments: –

Take the mean of multiple measurements – Repeat and compare results For collecting data: – –

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Integrate info from many reliable sources The reliability of a source can be assessed by comparing it to several other sources (reputable). Random errors are reduced by taking the mean of multiple measurements All systematic errors should be eliminated or occur to the same extent in each of a series of measurements All variables are kept constant

 All errors are reduced Second hand information:  How was the information gathered  The degree to which the data relates to the hypothesis or problem Systematic error: a carried on error through a series of measurements ( this error occurs to the same extent in each measurement) (eg needle on voltmeter is not at zero when there is no voltage) Random error: an error due to the variation in measurement technique